This invention relates to power amplifier control techniques and more particularly to methods and apparatus for controlling the output of a power amplifier as a function of the flux induced in a highly inductive load such as a gradient coil of an NMR imaging device. The invention achieves eddy current compensation while permitting an amplifier to properly handle both high bandwidth sinusoidal and pulse waveforms.
The new gradient sequence techniques employed for NMR imaging are presently of great interest to NMR scientists in that the same allow NMR images to be formed in a few minutes to under a minute rather than the tens of minutes usually required. Here, selected sequences of pulses and sinusoids are supplied to the gradient coil of an NMR device to cause rephasing of the proton spin instead of the more conventional approach of rephasing utilizing an R.F. pulse applied to the R.F. coil. These gradient sequence techniques, while being highly advantageous from the standpoint of the time required for image production and a marked reduction in the amount of R.F. to which is used and to which a patient is exposed to, imposed substantial operating hardships on the power amplifiers employed to drive the gradient coil in an NMR device. These hardships are often beyond the capability of the driving circuitry employed.
More particularly, most power amplifiers employed to drive the gradient coil in NMR devices operate as current mode output amplifiers wherein the output current produced is an exact representation of the voltage input supplied to the control circuit thereof. This mode of operation has been generally preferred since the function of the gradient coil in an NMR device is to produce a flux or field and its is know that flux produced is directly proportional to the current if no eddy currents are present.
Eddy currents and particularly those induced outside the wires of the power circuit represent a substantial problem in association with the gradient coils of NMR devices since eddy current effectively act to oppose the type of field which is desired to be produced. Further, if attempts are made to switch the field too fast, a situation can occur where no instantaneous field is present since the nearly equal and opposite effects of the field produced by eddy currents can act to cancel the field that is sought to be produced. This is particularly a problem where fast gradient sequence techniques are employed.
If the eddy currents produce a field that is spatially distributed in the same manner as the field produced by a gradient coil, attempts to make the signal provided to the coil compensating may be implemented by feedback control or through digital or analog signal predistortion techiques. With signal predistortion techniques, the input voltage to the amplifier is deliberately distorted so that the current supplied to the gradient coil will generate components of flux to compensate for the fields generated by the eddy currents induced. Such predistortion techniques are usually developed on an empirical basis and must be varied in accordance with the nature of the signal which is selected for application to the gradient coil.
Where gradient sequence techniques involving pulses and/or sinusoidal sequences are involved, empirical approaches to predistorting the input information supplied to the power amplifier driving the gradient coil can be tedious and time consuming. This is present even when time constants for dealing with the compensation function are provided by way of computer. Full compensation is rarely available because the flow of eddy currents is not generally limited to anticipated locations due to the non-optimum configuration of conducting surfaces where eddy currents are produced. Therefore, some eddy current fields are usually produced which are spatially incorrect and cannot be compensated.
The problem with eddy currents in NMR devices is so substantial that many designers have attempted to create a passive eddy current shield where conducting surfaces are provided in an effort to deliberately induce eddy currents having know spatial characteristics to enable the equal and opposite fields produced thereby to be cancelled by way of compensation. These, techniques typically attempt to create an ideal mirror for eddy currents by providing a conducting cylinder having the characteristics of infinite length so that the currents which are caused to flow are a perfect mirror of the currents in the coil. Infinite length characteristics are achieved by placing the conducting close to the coil. Thus, here all eddy currents which are induced are induced in the positive shield formed by the cylinder and therefore, in theory, may be compensated for by predistortion of currents launched in the gradient coil.
Those of ordinary skill in the art will appreciate that such predistortion techniques increase the current required to be produced by the power amplifier driving the gradient coil. Further, where a passive shield in the form of an infinite cylinder is employed, the characteristics of an infinite cylinder are generally achieved by placing the cylinder structure close to the coil. Thus further increases the magnitude of the eddy currents produced and hence, the magnitude of the current components which must be produced by the power amplifier to achieve predistortion. Thus, the whole approach to predistorting the input waveform to the power amplifier to compensate for the opposing fields produced by eddy currents, is one which inherently relies on increasing the current provided by the power amplifier.
Current mode output amplifiers generally employed to drive the gradient coils in NMR imaging devices have compromised design which is optimized in terms of providing output sequences which are either sinusoidal in nature or pulsed. This is necessary since in a current mode output amplifier a feedback series resistor is typically provided within the amplifier to guarantee that the current is an exact representation of the input control voltage signal. Thus, while the output impedance of a current mode output amplifier is typically very high, the resistance of the series resistor is quite low. This means that the voltage produced by the amplifier is exactly across the load, which in the case of a gradient coil, is almost purely inductive. As a result, the amplifier must frequently produce both maximum current and maximum voltage simultaneously with resistance of load reflected by the gradient coil generally being less than 1 ohm since, the amplifier is driving an essentially inductive load.
For this reason when the full voltage of the amplifier is to be provided in association with the output of sinusoidal signals, a resistance is frequently provided in series with the load to provide a location for part of the voltage across the output of the amplifier to drop so that the heat associated therewith may be dissipated. This is appropriate because with a sinusoidal output more time is generally required at higher power levels so that a resistor is needed to dissipate both voltage and heat. Conversely under sinusoidal output waveform conditions, if no resistor is provided in series with the load, heat must be dissipated within the amplifier causing the same to heat badly. Thus a design selection which employs a series resistor is prefectly acceptable when sinusoidal output information is to be provided in that the same allows full bandwidth while the deleterious effects of heating in the amplifier are avoided.
Use of a resistor in series with a gradient coil load is not acceptable in cases where the output of the power amplifier takes the form of pulses in the shape of square waves or the like. Here, what is then desired is that the pulses be able to rise as fast as possible. This means full signals is to be applied across the load as quickly as possible. Hence, no series resistor is desirable because the full voltage of the amplifier would not be available across the load and fast rise times, which are required in the case of rapid pulse gradient methods, would not be achieved. Accordingly, when a current mode output amplifier is to provide pulse information to a gradient coil, use of a resistor in series with the load is either reduced or avoided since the full voltage of the amplifier across the gradient coil is required. This too is an acceptable design since the nature of the pulse information involved reduces the heat which must be dissipated by the amplifier from that present when sinusoids are employed.
The new gradient sequence techniques employed by NMR scientists, however, often require that both pulse and sinusoidal sequences be utilized so that conventional design compromises adapted to accommodate either pulses or sinusoids in current mode output amplifiers are not advantageous. Thus, unless switching in and out of the resistor in series with the gradient coil is utilized in association with a programming of the sequences to be output by the power amplifier, the compromised design criteria may not be employed. Such switching, however, is of dubious utility since appropriate current/high voltage switches for this purpose are expensive and difficult to implement. Furthermore, imposing amplifier switching requirements on an NMR scientist experimenting or developing new gradient sequencing techniques is onerous since such an experimenter should only be concerned with the attributes of a particular sequence being designed and not the requirements of the power amplifier for portions of a sequence involving pulses and sinusoids.
Therefore, it is a principal object of the present invention to provide improved methods and apparatus for controlling power amplifiers driving highly inductive loads such as the gradient coils in NMR imaging devices.
A further object of this invention is to provide methods and apparatus for controlling power amplifiers driving highly inductive loads to automatically compensate for eddy currents induced in surrounding structure.
An additional object of the present invention is to provide methods and apparatus for controlling amplifiers delivering both high bandwidth sinusoids and pulse waveforms to a highly inductive load.
Various other objects and advantages of the present invention will become clear from the following detailed description of an exemplary embodiment thereof and the novel features will be particularly pointed out in conjunction with the claims appended hereto.